diag
From base v3.3
by Rcore Rcore@Rproject.org
Matrix Diagonals
Extract or replace the diagonal of a matrix, or construct a diagonal matrix.
 Keywords
 array
Usage
diag(x = 1, nrow, ncol)
diag(x) < value
Arguments
 x
 a matrix, vector or 1D array, or missing.
 nrow, ncol
 Optional dimensions for the result when
x
is not a matrix.  value
 either a single value or a vector of length equal to that
of the current diagonal. Should be of a mode which can be coerced
to that of
x
.
Details
diag
has four distinct usages:

x
is a matrix, when it extracts the diagonal. 
x
is missing andnrow
is specified, it returns an identity matrix. 
x
is a scalar (lengthone vector) and the only argument, it returns a square identity matrix of size given by the scalar. 
x
is a numeric vector, either of length at least 2 or there were further arguments. This returns a matrix with the given diagonal and zero offdiagonal entries.
It is an error to specify nrow
or ncol
in the first case.
Value

If x is a matrix then diag(x) returns the diagonal of
x. The resulting vector will have names if the
matrix x has matching column and rownames.The replacement form sets the diagonal of the matrix x to the
given value(s).In all other cases the value is a diagonal matrix with nrow
rows and ncol columns (if ncol is not given the matrix
is square). Here nrow is taken from the argument if specified,
otherwise inferred from x: if that is a vector (or 1D array) of
length two or more, then its length is the number of rows, but if it
is of length one and neither nrow nor ncol is specified,
nrow = as.integer(x).When a diagonal matrix is returned, the diagonal elements are one
except in the fourth case, when x gives the diagonal elements:
it will be recycled or truncated as needed, but fractional recycling
and truncation will give a warning.
Note
Using diag(x)
can have unexpected effects if x
is a
vector that could be of length one. Use diag(x, nrow =
length(x))
for consistent behaviour.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
See Also
Examples
library(base)
require(stats)
dim(diag(3))
diag(10, 3, 4) # guess what?
all(diag(1:3) == {m < matrix(0,3,3); diag(m) < 1:3; m})
diag(var(M < cbind(X = 1:5, Y = stats::rnorm(5))))
#> vector with names "X" and "Y"
rownames(M) < c(colnames(M), rep("", 3));
M; diag(M) # named as well
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